to check the torsion bar, put a floor jack under the rear differential and jack the rear tires off the ground. You adjust the tension on the torsion bar via a bolt into a "key" at the cross member. Most people will measure the how much of the bolt is exposed - normal is like 1 1/2" exposed. They start there and see how far off level the vehicle is without the rear tires influencing the amount of tension in the torsion bar. If you need to adjust the bolt, you MUST jack the front tires off the ground. If you do not, you WILL strip the bolt... not expensive to fix; however, it's a top 20 PITA. You reduce the amount of the bolt exposed to increase the tension (tighten the bolt).
For the rear springs - you'll looking for dissimilar shapes. the rear springs tend to break/bend right where the front of the spring makes the eye that captures the front bushing. If that is the case, replace the springs with new ones.... don't bother with used. You can also jack the front up to see if the rear springs aren't holding the vehicle level - but it's harder to do because centering the jack isn't as simple as it is in the back. In both front and rear, you want just a touch more tension on the driver's side than the passenger side because most times the passenger side is empty, but also, that's the side the fuel tank is on... once you've set the suspension, it should be just a touch leaning to the right (not enough to be noticeable, but enough if you measure to the bumper, you'll be 1/32" higher on the left side.
I suppose for completeness, if your bushing(s) are shot - that also would allow a lean... given you're in Mexico and heat happens there - it wouldn't be surprising if a bushing is shot. (the bushings in the front and rear of the leaf springs).
Something else to consider, if you run out of adjustment or you adjust it and it doesn't stay level - the torsion bars are springs that do wear out over time... and it's not uncommon for people here to have replaced the springs. If you do replace the front bars, the 2008 up Alpha springs are a higher rate and preferable (they cost the same).
Finally, be sure your shocks aren't leaking - that also can affect the ride and (sometimes) whether it stays level.
